exander gpio controller bindings

Required properties:
- compatible:
  - "xxxx,gpio-expander" for expander gpio controllers
- #gpio-cells : Should be two.
  - first cell is the pin number
  - second cell is used to specify optional parameters (unused)
- gpio-controller : Marks the device node as a GPIO controller.
- xxxx,gpio_start : Specify the start_number of exander gpios
- xxxx,ngpio : Specify the number of MSM GPIOs

Example:

	expander_gpios: xxxx@20 {/*chip_name@slave_addr*/
		commpatible = "xxxx,gpio-expander";
		gpio-controller;
		#gpio-cells = <2>;
		xxxx,gpio_start = <300>;
		xxxx,ngpio = <16>;
	};

To specify gpios for a device:

	sample@12 {
		compatible = "test,sample";
                sample,expander_lcd_en = <&expander_gpios 4 0>;
                sample,exapnder_lcd_rst = <&expander_gpios 5 0>;
	};

4, 5 - gpio numbers.
The driver for sample can call of_get_gpio() to extract the
gpio4. In order to extract gpio5, the driver needs to call
of_get_named_gpio with "sample,exapnder_lcd_rst" as the name parameter.
Please refer to the file: include/linux/of_gpio.h for the
complete list of APIs the driver can use to extract gpio
information from the device tree.
